As long as there's no DRM, just connect both the thumb drive and the KT and drag the books to the Documents folder. Of course, they have to be in a format the Kindle supports, like MOBI or PDF. You should be able to convert them in your Calibre.

Kobo Touch uses epub format and Kindle Touch uses mobi format. Have you converted the books using calibre to mobi format already? Yes, using a computer you can sideload (drag and drop) the ebooks into the documents folder on the Kindle Touch.

There is a calibre portable program, you could put that and the calibre library both on a thumb drive and use that for managing the books without him having to download the calibre program.

One other thing to consider is if you are going to be loading a whole lot of books onto his Kindle for him, it's maybe worth doing it in small batches and not dumping 100 books straight on there.

If you do it in batches and let the Kindle index those books, check there's no problem and then load on your next batch, index, check and so on. Just in case you load them all on and one book has some kind of issue that causes the indexing to get stuck.

You can check the books are indexed ok by searching for a non-existent term like 'xywqr' and on the results page it tells you if any books are still to be indexed. If it gets stuck on one book, there's a problem with that particular book.

One other thing to consider is if you are going to be loading a whole lot of books onto his Kindle for him, it's maybe worth doing it in small batches and not dumping 100 books straight on there.

If you do it in batches and let the Kindle index those books, check there's no problem and then load on your next batch, index, check and so on. Just in case you load them all on and one book has some kind of issue that causes the indexing to get stuck.

You can check the books are indexed ok by searching for a non-existent term like 'xywqr' and on the results page it tells you if any books are still to be indexed. If it gets stuck on one book, there's a problem with that particular book.